Processes of petroleum refining, of chemical treatment and of production of organic compounds are well known wherein reagents in different phases, liquid and gaseous, are premixed and input into vertical reactors to react with one another during their passage in a continuous and descending flow through one or more layers of granular solid catalysts supported on permeable trays, intercalated in the vertical sense within such reactor, denominated fixed beds of catalysts.
Among petroleum-refining processes employing this technique are processes known as hydrotreatment, hydrogenation, dehydrogenation, hydrocracking, in addition to others. All thereof are processes generating exothermic reactions, the reagents whereof generally being liquid or partially-vaporised hydrocarbons and gaseous hydrogen.
For example, in a typical hydrocracking process a charge of liquid hydrocarbons of high molecular weight and initial boiling point (IBP) in a band from 320° C. to 390° C. is premixed with gaseous hydrogen and subsequently injected into the top of a typical vertical reactor, such as that schematically represented in FIG. 1 accompanying the present specification.
The charge flows down the reactor and on passing continuously through fixed beds of catalyst it reacts with itself generating lighter hydrocarbons having molecular weights in the diesel band (IBP approximately 30° C.) or in the petrol band (IBP approximately 100° C.), depending on the degree of severity imposed on the process.
With the objective of such exothermic reactions occurring in an efficient manner during such passage of charge through the fixed beds of catalyst it is fundamental: firstly, that the gaseous phase (hydrogen) remain well-mixed with the liquid phase (hydrocarbons) to ensure maintenance of the designed stoichiometric ratio of the reagents and, secondly, that said charge be uniformly distributed onto the surfaces of the fixed beds of catalyst. Uniform distribution prevents the formation of preferential routes, or hot points, which pass around (by-pass) parts of the catalyst present in said beds, whilst overloading others, experiencing higher acceleration of catalyst deactivation due to coke formation.
To address said two process requirements simultaneously is a complex task insofar as, not only is there a natural tendency towards separation of the phases of the charge, but in addition preferential flows, or channelling, tend to form in a random manner without apparent reason.
The art teaches employment of devices called distribution trays to resolve said aforementioned problems. Installed between charge inlet locations and the surfaces of the fixed beds, and occupying the entire transverse cross-section of the reactor, such devices were initially simple perforated plates, the function whereof was the creation of a protective shield which would not only prevent the charge falling onto a single point of the catalytic bed but which was also capable of distributing the flow of charge radially by means of diverse orifices. Such increase in the number of points of arrival of the charge on such beds of catalysts improves the distribution of the charge onto the bed.
Currently the majority of said devices basically comprise trays having the most varied forms installed in the same manner and locations as said perforated plates however, in contrast to orifices, being drained by means of several small dykes, similar to flues, assuming innumerable configurations. Said devices, in general tubular and provided with a cap having lateral apertures, allow gas to pass preventing the direct passage of liquid, it being partially retained on such tray forming a level reservoir, it subsequently overflowing, cascading within said tubular dykes together with the gas, promoting further contact between the two phases and better distribution of charge onto the surface of the catalytic beds.
Appearance of said new device increased the efficacy of the process by virtue of the more prolonged contact between the gaseous phase and the liquid phase retained on the trays, enhancing the homogeneity of the charge, however requirements for improvement in the technique of distribution of the latter were still necessary.